Stephen Colbert's appearance before Congress in Washington DC today produced the spectacle of a satirical character testifying on a serious subject – an uneasy alliance even in Washington DC, where fact and fiction are co-habiting bedfellows.

Colbert was testifying to the House of Representatives subcommittee on immigration, at the invitation of committee chair Zoe Lofgrun, as part of the "Take Our Jobs" campaign by the United Farm Workers of America union, highlighting the work done by migrant farm workers, which Colbert has featured on his Comedy Central show.

While immigration reform is a sensitive subject, Colbert's testimony was conducted in his television persona satirizing a conservative talkshow host. So Colbert's opening remarks went like this:

America's farms are far too dependent on immigrant labour to pick our fruits and vegetables. Now the obvious answer is for all of us to stop eating fruits and vegetables. And if you look at the recent obesity statistics, many Americans have already started. Unfortunately, my gastroenterologist has informed me in no uncertain terms that they are a necessary source of roughage. In evidence I would like to submit a video of my colonoscopy into the congressional record.

Colbert's appearance seemed to annoy the House Judiciary chairman John Conyers, who noted that Colbert's testimony differed markedly from his more serious written testimony submitted earlier. But Colbert correctly maintained that he was appearing as a guest of Lofgrun's, and so Conyers withdrew.

While amusing, Colbert's faux-Fox character did make for uncomfortable listening for those who worry about the slipping credibility of Congress among voters, especially when Colbert made statements such as this:

This is America - I don't want a tomato picked by a Mexican. I want it picked by an American. And then sliced by a Guatemalan and served by a Venezuelan in a spa where a Chilean gives me a Brazilian. Because my great-grandfather did not travel across 4,000 miles of the Atlantic ocean to see this country overrun by immigrants. He did it because he killed a man back in Ireland. That's the rumour, I don't know if that's true, I'd like to have that stricken from the record.

On the other hand, congressional committees regularly invite celebrity witnesses, in the knowledge that it will put their obscure subcommittee in the limelight if Brad Pitt, Angelina Jolie or George Clooney appears before it. In that respect Colbert is no different. And at least Colbert is funny, unlike Jolie, Clooney, et al.

In Colbert defence, he had previously covered the "Take Our Jobs" campaign on his show, as an illustration of the need for migrant labour since very few Americans would take up the campaign's offer of poorly-paid, backbreaking farming jobs.

In a recent episode Colbert spent a day working on a farm in upstate New York, which he described to Congress as "my vast experience spending one day as a migrant farm worker". It also led to a testy exchange with Steve King, an Republican congressman from Iowa who quibbled about TV footage of Colbert packing and unpacking boxes of vegetables. Colbert told him: "I was a corn packer. I know that term is offensive to some people, because corn packer is a derogatory term for a gay Iowan."

Colbert did also make a serious point, saying that he wanted to speak up for migrant farm workers because "migrant workers suffer and they have no rights." Even Conyers was impressed. "I thought he was pretty profound," Conyers said after the hearing.

Update: House majority leader Nancy Pelosi defended Colbert's appearance: "Of course I think it's appropriate. He's an American, right? He comes before the committee, has a point of view, he can bring attention to an important issue like immigration. I think it's great."